1. Field of the Invention
The present invention relates to a scanning exposure method of the step-and-scan type in which a mask and a substrate are synchronously scanned to transfer a mask pattern onto the substrate. More specifically, the invention relates to such a scanning exposure method in which the time between scans of the pattern is reduced.
2. Discussion of the Related Art
In a photolithography process for manufacturing a semiconductor or the like, a projection-type exposure apparatus uses a projection optical system to transfer an image of a pattern on a mask or a reticle to a photosensitive substrate. The substrate typically is a wafer or glass plate with photoresist applied thereto. During the process, a much larger pattern on the reticle is transferred to the wafer. For example, a reticle pattern may be about four times the size of the transferred image to the wafer. Various scanning-type exposure apparatuses have been developed in which the reticle and the wafer are scanned synchronously with respect to an illumination area, often a slit-like illumination area, to transfer the large pattern to the wafer.
More particularly, a wafer includes a plurality of shot areas or chips on which a reticle pattern is scanned and exposed. After scanning and exposing a reticle pattern onto a first shot area of a wafer, the wafer and reticle must be stepped to the next shot area and pattern, respectively, to begin the scanning of that next pattern. The stepping of the wafer positions the subsequent shot area to a scanning start position. Often, during the stepping of the wafer, the direction of scanning must be reversed. The stepping and scanning exposure is repeated for all shot areas on the wafer. This system of repeating the stepping and the scanning exposure is commonly called a step-and-scan system.
U.S. patent application Ser. No. 08/356,619, filed Dec. 7, 1994, discloses a step-and-scan system, and is hereby incorporated by reference. As shown at FIGS. 7(a), 7(b), and 7(c), the disclosed system illustrates a velocity wave form having linear velocity segments.
In the step-and-scan system, the stepping time between scans represents an inefficiency in which no scanning occurs. This inefficiency increases the total time to manufacture a chip on a wafer and thereby limits the throughput of wafers in a production process. The need therefore exists to minimize the stepping time between scans to increase wafer throughput.